Electronic circuit and low voltage arc flash system including an electromagnetic trigger

ABSTRACT

An electronic circuit includes a number of sensors structured to detect an arc flash from an uncontrolled arcing fault, and a trigger circuit, responsive to the detected arc flash, structured to trigger a triggering mechanism and cause a breakdown of a number of gaps within a low voltage arc flash switch.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to commonly assigned, copending U.S. patentapplication Ser. No. ______, filed ______, entitled “Low Voltage ArcFlash Switch” (Attorney Docket No. 12-mMCC-754).

BACKGROUND

1. Field

The disclosed concept pertains generally to arc flash mitigation and,more particularly, to trigger circuits for low voltage arc flashswitches. The disclosed concept also pertains to low voltage arc flashsystems including a number of trigger circuits.

2. Background Information

Arc flash mitigation is needed in low voltage power applications. Arcflash hazards are particularly dangerous when maintenance is performedon energized equipment (e.g., without limitation, motor-control centers(MCCs)). Often, service doors are opened during maintenance, whichincreases the likelihood of maintenance personnel getting injured ifthey make a mistake. Also, other dangerous arc flash situations caninvolve degraded insulation or animals creating shorts across energizedconductors.

There is room for improvement in low voltage arc flash systems.

SUMMARY

These needs and others are met by embodiments of the disclosed conceptin which a trigger circuit responds to a detected arc flash and triggersa triggering mechanism in order to cause a breakdown of a number of gapswithin a low voltage arc flash switch.

In accordance with one aspect of the disclosed concept, an electroniccircuit comprises: a number of sensors structured to detect an arc flashfrom an uncontrolled arcing fault; and a trigger circuit, responsive tothe detected arc flash, structured to trigger a triggering mechanism andcause a breakdown of a number of gaps within a low voltage arc flashswitch.

As another aspect of the disclosed concept, a low voltage arc flashsystem comprises: a low voltage arc flash switch including a number ofgaps within the low voltage arc flash switch; a number of triggeringmechanisms, one for each of the number of gaps; and an electroniccircuit comprising: a number of sensors structured to detect an arcflash from an uncontrolled arcing fault, and a trigger circuit,responsive to the detected arc flash, structured to trigger the numberof triggering mechanisms and cause a breakdown of the number of gapswithin the low voltage arc flash switch.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the disclosed concept can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is an isometric view of system including a low voltage arc flashswitch and a three-phase power bus in accordance with embodiments of thedisclosed concept.

FIG. 2 is a cross-sectional view of the low voltage arc flash switch andthe three-phase power bus along lines 2-2 of FIG. 1.

FIG. 3 is a top plan view of the low voltage arc flash switch and thethree-phase power bus of FIG. 1.

FIG. 4 is a vertical elevation view of the elongated conductive cylinderand support of FIG. 1.

FIG. 5 is an end elevation view of the elongated conductive cylinder andsupport of FIG. 4.

FIG. 6 is a cross-sectional view of one of the end caps of FIG. 1.

FIG. 7 is a cross-sectional view of a low voltage arc flash switch and athree-phase power bus in accordance with another embodiment of thedisclosed concept.

FIG. 8 is a plot of current waveforms including prospective currentwithout a low voltage arc flash switch and limited current with the lowvoltage arc flash switch of FIG. 7.

FIG. 9 is an isometric view of a pair of the metal contacts of FIG. 7.

FIG. 10 is a block diagram of an electronic circuit and a low voltagearc flash switch in accordance with another embodiment of the disclosedconcept.

FIGS. 11A and 11B are vertical elevation views of an electromagnetictrigger for the low voltage arc flash switch of FIG. 1 in respectivecompressed and triggered positions.

FIG. 11C is a top plan view of a copper ribbon for the electromagnetictrigger of FIG. 11A.

FIGS. 12A and 12B are vertical elevation views of electromagnetictriggers for the two gaps of the low voltage arc flash switch of FIG. 1in respective compressed and triggered positions.

FIGS. 13A and 13B are vertical elevation views of an electromagnetictrigger for the low voltage arc flash switch of FIG. 7 in respectivecompressed and triggered positions.

FIG. 14 is a block diagram in schematic form of a single-phase open doortrigger circuit for the electromagnetic trigger of FIG. 12A.

FIG. 15 is a block diagram in schematic form of a three-phase open doortrigger circuit for the electromagnetic trigger of FIG. 12A.

FIG. 16 is a block diagram in schematic form of a three-phase full-timeprotection trigger circuit for the electromagnetic trigger of FIG. 12A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As employed herein, the term “number” shall mean one or an integergreater than one (i.e., a plurality).

As employed herein, the statement that two or more parts are “connected”or “coupled” together shall mean that the parts are joined togethereither directly or joined through one or more intermediate parts.Further, as employed herein, the statement that two or more parts are“attached” shall mean that the parts are joined together directly.

The disclosed low voltage (i.e., less than 1000 V_(RMS)) arc flashswitch employs a triggering mechanism, such as an electromagnetictrigger or fusible link, to trigger the device. The example triggeringmechanism causes a breakdown of a gap between conductors in a sealedhousing, which can optionally be pressurized. The switch includessuitably high melting point metal conductors enclosed in a sealedcontainer structured to contain an arcing fault. Upon detection of anuncontrolled external arcing fault, the example triggering mechanism isinitiated which causes the external arcing fault to commutate into thesealed switch, thereby eliminating the external arcing fault andprotecting personnel and equipment from arcing damage.

Referring to FIGS. 1-3, a low voltage power system 2 includes a lowvoltage arc flash switch 4 and a three-phase low voltage power bus 6.The low voltage arc flash switch 4 includes a sealed housing 8, gasinsulation 10 (FIG. 2) within the sealed housing 8, a plurality (e.g.,without limitation; two; three; greater than three) of conductors 12(FIG. 2) including a number (e.g., without limitation; one; two; greaterthan two) of gaps 14 (FIG. 2) therebetween within the sealed housing 8,and a triggering mechanism 15 (shown in FIG. 10 in connection with asingle gap 138), structured to cause a breakdown of the number of gaps14.

The example three-phase low voltage power bus 6 includes three lowvoltage power bus bars 16, which are optionally supported by insulative(e.g., without limitation, GP03; red glass) support braces 18. The threelow voltage power bus bars 16 carry a three-phase alternating currentlow voltage, which is received by the three example conductors 12 shownin FIG. 2.

As will be discussed, the example triggering mechanism 15 places anarcing fault across the three conductors 12 within the sealed housing 8,in order to eliminate an uncontrolled arcing fault (e.g., withoutlimitation, phase-to-phase; phase-to-ground) external to the sealedhousing 8. For example, the triggering mechanism 15 is structured tocause the uncontrolled arcing fault external to the sealed housing 8 tocommutate into the sealed housing 8, thereby eliminating theuncontrolled arcing fault.

For example and without limitation, a first voltage across each of thetwo example gaps 14 shown in FIG. 2 is about 25 V_(RMS) to about 120V_(RMS). Before the uncontrolled arcing fault, a second voltage betweentwo phases of the example three-phase alternating current low voltage isany suitable low voltage (e.g., without limitation, typically about 208V_(RMS) to about 690 V_(RMS); any suitable low voltage less than 1000V_(RMS) or less than 1500 V_(DC)). Fault currents from the arcing faultacross the three example conductors 12 are conducted within the sealedhousing 8 for up to about 30 line cycles. Since the low voltage arcflash switch 4 is a sealed unit, no exhaust gas escapes from the sealedhousing 8 responsive to the uncontrolled arcing fault commutated intothe sealed housing 8. The uncontrolled arcing fault is advantageouslycommutated in under 3 ms. Hence, the arcing fault is commutated into thesealed container 8 across all three example phases, thereby eliminatingthe arc flash hazard in under 3 ms. Fault currents can be contained forup to about 30 cycles. This results in a significant current limiting(e.g., about a 20% to 40% reduction) thereby protecting upstreamequipment from thermal and mechanical stress.

As a non-limiting example, the example conductors 12 are made oftungsten. The gas insulation 10 is a number (e.g., one gas; a mixture ofgasses) of gasses selected from the group consisting of nitrogen,hydrogen, argon, sulfur hexafluoride, helium, and air. The gasinsulation 10 has a nominal quiescent pressure within the sealed housing8 of between 10⁻⁷ Torr and 10⁴ Torr, where one atmosphere is equal to760 Torr (101,325 Pa.).

As shown in FIGS. 3 and 4, the example sealed housing 8 includes anelongated conductive cylinder 20 having a first end 22, an intermediateportion 24 and an opposite second end 26. A first conductive end cap 28(FIG. 3) is coupled to the first end 22, and a second conductive end cap30 (FIG. 3) is coupled to the opposite second end 26. The cylinder 20and the end caps 28,30 can be made, for example and without limitation,of stainless steel. The end caps 28,30 are preferably brazed, welded orthreaded to the respective ends 22,26 of the elongated conductivecylinder 20.

A first one (e.g., without limitation, phase A) of the three conductors12 passes through the first conductive end cap 28 and is structured tobe electrically and mechanically coupled to a first bus bar 32 energizedby a first phase of the three-phase alternating current low voltage. Asecond one (e.g., without limitation, phase B) of the three conductors12 passes through the intermediate portion 24 of the elongatedconductive cylinder 20 and is structured to be electrically andmechanically coupled to a second bus bar 34 energized by a second phaseof the three-phase alternating current low voltage. A third one (e.g.,without limitation, phase C) of the three conductors 12 passes throughthe second conductive end cap 30 and is structured to be electricallyand mechanically coupled to a third bus bar 36 energized by a thirdphase of the three-phase alternating current low voltage.

As shown in FIG. 6, each of the example end caps 28,30 includes an outerconductive end cap portion 38 electrically and mechanically coupled to acorresponding one of the ends 22,26 of the elongated conductive cylinder20, and an inner insulator member 40 carrying an inner conductiveportion 42 (FIG. 2) (e.g., without limitation, made of copper; steel) ofa corresponding one of the first and third ones (e.g., withoutlimitation, phases A and C) of the three conductors 12. A number (e.g.,without limitation, two are shown in FIG. 6) of axial O-ring seals 44seal a first surface 46 of the inner insulator member 40 to a firstsurface 48 of the outer conductive end cap portion 38. A compressionO-ring seal 50 (e.g., without limitation, made of Viton® syntheticrubber) seals a second surface 52 of the inner insulator member 40 to asecond surface 54 of the outer conductive end cap portion 38. Theexample seals 44,50 seal the outer conductive end cap portion 38 to theinner insulator member 40.

As shown in FIG. 2, an insulative disk 56 (e.g., without limitation,made of alumina; Macor® glass-ceramic material; silicon carbide) withinthe sealed housing 8 encloses the inner conductive portion 42 of thecorresponding first and third conductors 12 and the inner insulatormember 40 within the outer conductive end cap portion 38 and away fromthe two example gaps 14 within the sealed housing 8. Each of the firstand third conductors 12 includes an angled conductive bracket 58 (e.g.,without limitation, made of copper) having a first portion 60electrically coupled to a corresponding one of the first and thirdconductors 12 and a second portion 62 structured to be electricallycoupled to a corresponding one of the first and third bus bars 32,36.Each of the conductive end caps 28,30 further includes an insulativedisk 64 (e.g., without limitation, made of a G10 glass reinforced epoxy)outside of the sealed housing 8 enclosing the inner conductive portion42 of the corresponding one of the first and third conductors 12 and theinner insulator member 40 within the outer conductive end cap portion 38and away from exterior surface 66 of the conductive end cap portion 38.

The insulative disk 56 advantageously protects the inner conductiveportion 42 and the other insulative disk 64. The insulative disk 64advantageously provides insulation for a suitable over surface distance(e.g., between the B-A phases or between the B-C phases since phase B iselectrically connected to the elongated conductive cylinder 20 and theend caps 28,30). A number of nuts 68 (e.g., without limitation, made ofbrass; two are shown) are threaded on an outer conductive portion 70(e.g., without limitation, made of copper) of each of the first andthird conductors 12 to secure the insulative disk 64 to a correspondingone of the first and second conductive end caps 28,30. This compressesthe compression O-ring seal 50 (FIG. 6), and secures the angledconductive bracket 58 to the corresponding one of the first and thirdconductors 12. Although an inner nut 68 is shown for locking purposesand cinching the compression O-ring seal 50, only the outer nut 68 isneeded. The outer second nut 68 allows the switch 4 to be bolted to thebracket 58 (phase A or phase C) if the spacing between the inner firstnut 68 and the outer second nut 68 needs to be adjusted.

Each of the first and third conductors 12 includes an inner conductiveelectrode 72 (shown in FIG. 5 with the B-phase conductor) (e.g., withoutlimitation, made of tungsten; steel; copper; copper-chrome) within thesealed housing 8 and the external conductor or outer conductive portion70 (FIGS. 1-3) (e.g., without limitation, made of copper) outside of thesealed housing 8. The inner conductive electrode 72 is brazed to theinner conductive portion 42 which is part of the external conductor 70.As a further non-limiting example, the inner conductive electrode 72 isadvantageously made of tungsten, in order to provide a suitably highmelting point, to reduce vapor pressure from relatively low erosion ofthe tungsten, and to slow pressure build up within the sealed housing 8.In the example of FIG. 2, the inner conductive electrode 72 of thesecond one of the three conductors 12 is normal to both of the innerconductive electrodes 72 of the second and third ones of the threeconductors 12.

Continuing to refer to FIG. 2, an external conductive (e.g., withoutlimitation, made of copper) support and mounting member 74 is brazed toan exterior surface 75 of the elongated conductive (e.g., withoutlimitation, made of stainless steel) cylinder 20. The second one of thethree conductors 12 is preferably made of tungsten and is brazed to theelongated conductive cylinder 20 at locations 21 and to the externalconductive support and mounting member 74 at locations 73 as best shownin FIG. 5.

The external conductive support and mounting member 74 includes agenerally planar conductive surface 76 having a first width W1structured to be electrically and mechanically coupled to the second busbar 34, which is an elongated rectangular conductive bus bar having alarger second width W2. The second bus bar 34 is energized by the secondphase of the three-phase alternating current low voltage. The generallyplanar conductive surface 76 includes a recess 78 having a third widthW3, which is smaller than the first width W1, structured to receive aninsulative planar barrier 80 (e.g., without limitation, fish paper)therein. The structure of the insulative planar barrier 80 and theresulting current flow helps to retain the arcs in the gaps 14 for theA-B phases and the B-C phases. The larger second width W2 of the secondbus bar 34 allows for a reverse current loop. Current flows from themating conductive surfaces and travels laterally (with respect to FIG.2) towards the center of member 74. Current continues to flow vertically(with respect to FIG. 2) through the center electrode 72 across the arcformed in the gap between electrodes 72 and conductors 12 and laterally(with respect to FIG. 2) through conductors 12. This creates a magneticfield which tends to keep the arc in the gap formed between conductors12 and electrode 72 and also tends to drive the arc upward (with respectto FIG. 2).

Referring again to FIGS. 4 and 5, an interior of the example stainlesssteel elongated conductive cylinder 20 is covered with a suitablethermal ceramic spray 83, which protects the conductive cylinder 20 fromarcing therein. Also, as shown in FIG. 5, an internal portion of theexample second tungsten conductor 12 can also be covered with thethermal ceramic spray 83, leaving a suitable portion (shown hatched)uncovered, which forms the inner conductive electrode 72.

As shown in FIG. 3, the elongated conductive cylinder 20 optionally hasa number of U-clamps 82 (e.g., without limitation, made of steel)structured to electrically and mechanically couple to the second bus bar34. These advantageously stiffen and avoid mechanical stresses, such asbending, of the bus bar 34.

FIG. 7 shows another low voltage arc flash switch 84, which is somewhatsimilar to the low voltage arc flash switch 4 of FIGS. 1-3, and athree-phase power bus 86 (having phases A, B, C). For example andwithout limitation, in FIG. 7, one vertical (with respect to FIG. 7) rod88 and two horizontal (with respect to FIG. 7) rods 90,92 are made ofcopper and two sets 94,96 of example tungsten contacts 98 are provided.It is believed that this configuration increases the current (e.g.,without limitation, from 35 kA to 65 kA with respect to theconfiguration of FIG. 1) and increases the time duration of arcing(e.g., without limitation, 3 to 30 cycles) while maintaining theintegrity of elongated conductive cylinder 100 (e.g., withoutlimitation, made of stainless steel).

Preferably, in this example, a different electrode geometry is employed.The addition of the example tungsten contacts 98 formed by the exampletungsten contact disks 102 (shown in FIG. 9) provides a relativelylarger surface area to reduce arc erosion and, more importantly,preferably employs known vacuum interrupter contact technology to rotatethe arc to further minimize arc erosion of the contacts 98 as well asdistribute thermal loading on the elongated conductive cylinder 100.This changes the magnetic fields and the current path. As shown in FIG.7, the current path is directed axially along the horizontal rods90,91,92 disposed in the center of the elongated conductive cylinder 100to the center 99 of the tungsten contacts 98 (FIG. 9). From that point,the current moves outward on a contact pedal 104 (FIG. 9) and returns onthe adjacent contact pedal 106 (FIG. 9), thereby forming a reverse loopthat creates a circumferential force on the arc at each of the two gaps108,110 to rotate the arc around the periphery of each of the contacts98. Optionally, a ferrous steel disk (not shown) can be employed behindeach tungsten contact 98 for increasing the magnetic force on the arc.The two sets 94,96 of four tungsten contacts 98 are structured to formthe reverse current loop.

FIG. 8 shows a plot of current waveforms including the prospectivecurrent without the low voltage arc flash switch 84 (FIG. 7) and theresulting current-limiting (CL) effect with the low voltage arc flashswitch 84. Current-limiting is desired to reduce system stress (e.g.,mechanical and thermal) and maintain arc current for a suitable numberof cycles with the example gaps 108,110 (FIG. 7) and correspondingmaterials, as disclosed.

FIG. 9 shows two of the tungsten contacts 98 including the pedals104,106. These contacts 98 are generally disk-shaped with a plurality ofgenerally L-shaped arms 112 forming the contact pedals 104,106 and beingstructured to rotate an arc at each of the two gaps 108,110 (FIG. 7).

Referring again to FIG. 7, a first one of the three conductors 114 isT-shaped and includes a first portion 116 normal to both a second oneand a third one of the three conductors 114, and a second portion 118in-line with both the second one and the third one of the threeconductors 114. The second portion 118 includes a first contact 120 at afirst end 122 thereof and a second contact 124 at an opposite second end126 thereof. The second one of the three conductors 114 includes a thirdcontact 128 facing the first contact 120 and forming the first gap 108.The third one of the three conductors 114 includes a fourth contact 130facing the second contact 124 and forming the second gap 110.Preferably, the contacts 120,124,128,130 are made of tungsten, and thethree conductors 114 are otherwise made of copper or steel.

As shown in FIG. 10, another low voltage arc flash switch 134 includestwo conductors 136, and one gap 138 therebetween. The two conductors 136are structured to receive a single phase alternating current lowvoltage. Otherwise, the low voltage arc flash switch 134 can be somewhatsimilar to the low voltage arc flash switch 4 of FIG. 1.

The electronic circuit 140 can be on board or at or near the low voltagearc flash switches 4,84,134. Optical and current sensors 142,144 detectan external arc flash 146 and trigger the electronic circuit 140 toclose the low voltage arc flash switch 4,84,134.

As an alternative to the thermal ceramic spray 83 of FIGS. 4 and 5, theinterior of the example stainless steel elongated conductive cylinder 20of FIG. 1 can be formed by a graphite tube or a ceramic tube that actsas an arc shield and protects the conductive cylinder 20 from arcingtherein, such as from a direct arc blast from contacts formed by theconductors 12 (FIG. 2) at the gaps 14 (FIG. 2).

In FIG. 10, the electronic circuit 140 includes the number of sensors142,144 that detect the arc flash 146 from an uncontrolled arcing fault,and a trigger circuit 148 that triggers a number of triggeringmechanisms 15 and causes a breakdown of the number of gaps 138 withinthe low voltage arc flash switch 134. A low voltage arc flash system 150includes the low voltage arc flash switch 134, and the electroniccircuit 140. The electronic circuit 140 can be disposed on, at or nearthe low voltage arc flash switch 134. The number of sensors 142,144 canbe a plurality of sensors including a number of optical sensors 142 anda number of current sensors 144, as will be discussed, below, inconnection with FIGS. 14-16.

As will be described, below, in connection with FIGS. 11A-11C, 12A-12Band 13A-13B, the example number of triggering mechanisms 15 can beexpandable electromagnetic triggers as will be described.

FIGS. 11A and 11B are vertical elevation views of an expandableelectromagnetic trigger 152 for the low voltage arc flash switch 4 ofFIG. 1 in respective compressed and triggered positions. A suitableconductor, such as an example copper ribbon or foil 154 (FIG. 11C), isaccelerated across a gap 156 in order to breakdown that gap providing aswitching action on the order of 800 microseconds. This provides fastand reliable triggering for the low voltage arc flash switch 4. In thisexample, copper ribbon and copper foil behave in a like manner in termsof electromagnetic repulsion, although a copper ribbon may not be a wideas a copper foil.

FIG. 11C shows the example copper ribbon or foil 154. As a non-limitingexample, the copper ribbon or foil 154 has a width of about 0.1 inch(about 0.254 cm), a thickness of about 0.003 inch (about 0.00762 cm) anda height of about 0.325 inch (about 0.8255 cm). In this example, thecopper ribbon or foil 154 has an accordion shape, which can extendfurther than a single looped conductor. The current/voltage from thetrigger circuit 148 (FIG. 10) causes the copper ribbon or foil 154 tomove from the compressed state (FIG. 11A) to the triggered state (FIG.11B) as a result of electromagnetic repulsion. For example, folding thecopper ribbon or foil 154 back on itself creates a “reverse” loop whichcauses the plural conductor folds to repel one another when a suitablecurrent pulse is applied. The dimensions of the copper ribbon/foil 154are preferably selected to achieve sufficiently small mass andstiffness, and sufficiently large current carrying cross sectional area,in order to achieve full displacement across the gaps 156,158 in asufficiently short time prior to exceeding the thermal capability of theribbon/foil (resulting in melting of the ribbon/foil).

In the example of FIGS. 11A-11B, number of gaps 138 (FIG. 10) are twogaps 156,158. As shown in FIG. 11A, there are a first plurality of folds160 disposed within the first gap 156 and a second plurality of folds162 disposed within the second gap 158. Each of the first plurality offolds 160 and the second plurality of folds 162 has a compressedposition (FIG. 11A) before the ribbon or foil 154 is triggered by thetrigger circuit 148. Also, each of the first plurality of folds 160 andthe second plurality of folds 162 has a triggered position (FIG. 11B)after the conductive ribbon or foil 154 is triggered by the triggercircuit 148. The triggered position (FIG. 11B) causes the firstplurality of folds 160 to expand and breakdown the first gap 156, andthe second plurality of folds 162 to expand and breakdown the second gap158. These breakdowns preferably occur in about 800 microseconds afterthe trigger circuit 148 triggers the example electromagnetic trigger152. For example and without limitation, each one of both of: (a) thefirst plurality of folds 160 and (b) the second plurality of folds 162can include twelve folds and forms an accordion shape.

The trigger circuit 148 outputs a current pulse to the exampleconductive ribbon or foil 154. Current flowing through each of the firstplurality of folds 160 and the second plurality of folds 162 causes thefirst plurality of folds 160 to electromagnetically repel each other andcauses the second plurality of folds 162 to electromagnetically repeleach other, thereby causing the conductive ribbon or foil 154 to movefrom the compressed position (FIG. 11A) to the triggered position (FIG.11B).

Referring to FIGS. 12A and 12B, electromagnetic triggers 164,166 for thetwo gaps 14 of the low voltage arc flash switch 4 of FIG. 1 are shown inrespective compressed and triggered positions. These gaps 14 are formedby a first electrode or contact 12 separated from a second electrode orcontact 12. It will be appreciated that these electromagnetic triggers164,166 and the electromagnetic trigger 152 of FIGS. 11A-11B can alsofunction for the low voltage arc flash switch 84 of FIG. 7. A triggeringmechanism, such as the electromagnetic triggers 164,166, includes, foreach of the number of gaps 14, a U-shaped foil or ribbon conductor 168including a first end 170, a first elongated portion 172, a U-bend 174,a second elongated portion 176, an arcuate bend 178 and a second end180. As shown in FIG. 12A, the second end 180 is electrically connectedto a first electrode 182 and the first elongated portion 172 is parallelto the second elongated portion 176 and separated therefrom by a firstinsulator 184. The second elongated portion 176 is parallel to the firstelectrode 182 and separated therefrom by a second insulator 186. Thetriggering mechanism has a compressed position (FIG. 12A) before thetriggering mechanism is triggered by the trigger circuit 148 (FIG. 12A),and has a triggered position (FIG. 12B) after the triggering mechanismis triggered by the trigger circuit 148. The first end 170 and the firstelongated portion 172 are distal from the second electrode 188 in thecompressed position (FIG. 12A), and the first elongated portion 172electrically engages the second electrode 188 in the triggered position(FIG. 12B).

In this example, the U-shaped foil or ribbon conductor 168 is made ofcopper and has a thickness of about 0.003 inch (about 0.00762 cm). Thetrigger circuit 148 outputs a current pulse to the U-shaped foil orribbon conductor 168. Current flows in opposite directions through thefirst electrode 182 and the first elongated portion 172 and through thefirst elongated portion 172 and the second elongated portion 176 causesthe first electrode 182 to electromagnetically repel the first elongatedportion 172 and causes the first elongated portion 172 toelectromagnetically repel the second elongated portion 176. This causesthe gaps 14 to breakdown. The electrodes 182,188 are made from, forexample and without limitation, tungsten, copper, copper-chrome, orsteel. The dimensions of the copper ribbon/foil 168 are preferablyselected to achieve sufficiently small mass and stiffness, andsufficiently large current carrying cross sectional area, in order toachieve full displacement across the gaps 14 in a sufficiently shorttime. In another case, the ribbon may break during current flow butmomentum will carry the ribbon across the gap 14.

FIGS. 13A and 13B show another electromagnetic trigger 192 for the lowvoltage arc flash switch 84 of FIG. 7 in respective compressed andtriggered positions. In this example, there are four trigger conductors194 (FIG. 13B), with two trigger conductors 196,198 for each of the twoconductive foils or ribbons 200,202, respectively. Here, the foils orribbons 200,202 are completed insulated from the B-phase conductor 204,thereby ensuring that the trigger current passes through the ribbons200,202 in parallel. Also, there is one folded piece of conductiveribbon as opposed to multiple folds. This is easy to construct and isfaster than the relatively larger electromagnetic trigger 152 of FIGS.11A-11B, which has more folds. The example electromagnetic trigger 192has relatively less moving mass and a notch 205 provides a definitebreak point.

Each of the gaps 108,110 is formed by a first electrode 120,124separated from a second electrode 128,130. A triggering mechanism 218includes, for each of the gaps 108,110, the foil or ribbon conductor200,202 including a first end 220 electrically connected to the firstelectrode 120,124, an elongated portion 222 and a free second end 224,with the notch 205 formed in the elongated portion 222 proximate thefree second end 224. The elongated portion 222 is parallel to the firstelectrode 120,124 and separated therefrom by an insulator 226 in anon-triggered position (FIG. 13A). The triggering mechanism 218 has afirst position (FIG. 13A) parallel to the first electrode 120,124 beforethe triggering mechanism 218 is triggered by a trigger circuit 219, suchas the trigger circuit 148 of FIG. 10. The triggering mechanism 218 hasa triggered position (FIG. 13B) after the triggering mechanism 218 istriggered by the trigger circuit 219. The foil or ribbon conductor200,202 is distal from the second electrode 128,130 in the non-triggeredposition. The elongated portion 222 electrically engages the secondelectrode 128,130 in the triggered position.

The trigger circuit 219 outputs a current pulse to or from the freesecond end 224 and from or to, respectively, the first electrode120,124. Current flowing in opposite directions through the elongatedportion 222 and the first electrode 120,124 causes the first electrodeto electromagnetically repel the elongated portion 222, break theelongated portion 222 at the notch 205, and cause the elongated portion222 to electrically engage the second electrode 128,130 in the triggeredposition.

In this example where there are the two gaps 108,110, the triggeringmechanism 218 includes, for each of the two gaps, a triggering member228. The trigger circuit 219 outputs a current pulse in parallel to thetrigger member 228 for each of the two gaps 108,110.

As shown in FIG. 14, for example, the sensor 142 of FIG. 10 is a currentsensor 230. The trigger circuit 148 (FIG. 10) and the trigger circuit219 (FIGS. 13A-13B) can include a full-wave bridge 232 including anoutput 234 and an input 236 electrically connected to the current sensor230, a capacitor 238 electrically connected to the output 234 of thecurrent sensor 236, and an electronic circuit 240 structured to respondto a predetermined voltage across the capacitor 238 and output a currentpulse through the corresponding electromagnetic trigger 164,166 (FIGS.12A-12B) or triggering mechanism 218 (FIG. 13A).

The example trigger circuit 219 is a single-phase open door triggercircuit for the electromagnetic triggers 164,166 of FIGS. 12A-12B. As anon-limiting example, the current sensor 230 is structured to charge thecapacitor 238 at a charge rate of about 2 kV/ms for a currentcorresponding to a suitable arc flash event. The example predeterminedvoltage is about 2 kV; and the capacitor 238 is charged to thepredetermined voltage in about 1 ms. The triggering mechanism isstructured to breakdown the number of gaps 14 in about 0.4 ms responsiveto the current pulse therethrough. A relay contact 242 is electricallyconnected between the current sensor 230 and the input 236 of thefull-wave bridge 232. The relay contact 242 is normally closed when aswitchgear door 244 is open. The current sensor 230 in this example is asingle current transformer (CT) structured to sense current flowing in asingle phase of switchgear 246. The example single current transformer230 can include, for example and without limitation, a 0.012 inch(0.03048 cm) laminated M4 silicon, steel C-core #27, having 300 turns of#16AWG with a 0.002 inch (0.00508 cm) air gap (not shown).

The example 2 kV/ms charge rate is based on the need to quickly chargethe capacitor 238 and the electronic circuit 240 in order to fire theelectromagnetic triggers 164,166. The faster the triggering members164,166 can activate, the more effective the low voltage arc flashswitch 4 of FIG. 1 becomes. The arc flash will be extinguished faster ifthe capacitor 238 can charge as fast as possible. The capacitor 238 ischarged to about 2 kV in about 1 ms, which establishes the above chargerate. After the 1 ms charge time, the trigger current pulse will startand move the electromagnetic triggers 164,166 in about 0.4 ms toactivate the low voltage arc flash switch 4. As a result, the arc faultwill then be commutated into the low voltage arc flash switch 4 in about1.4 ms for this example. There will be some additional commutation timeas well. This example trigger circuit 219 does not sense arc flash lightbut becomes active when the example switchgear door 244 is open.

Referring to FIGS. 15 and 16, other trigger circuits 248 and 250,respectively, are shown. Here, current transformers 252, connected in aWYE configuration, sense over-currents, include three outputs254,256,258 and are structured to sense currents flowing in three phasesof switchgear (not shown). Also, the input of a full-wave bridge 260 isthree discrete inputs 262,264,266 each of which is electricallyconnected to a corresponding one of the three outputs 254,256,258 of theWYE current transformer 252.

The trigger circuit 248 of FIG. 15 is a three-phase open door triggercircuit for the electromagnetic triggers 164,166 of FIG. 12A. Thistrigger circuit 248 is actively sensing current only when any switchgeardoor (not shown, but see the switchgear door 244 of FIG. 14) is open.Achieving a sufficient capacitor charge earlier (because of a fastercharging rate) allows the electromagnetic triggers 164,166 to beactivated earlier, and stops the arc flash event earlier. Thus, the arcflash energy is reduced by achieving a faster charging rate. In thisexample, each of three relay contacts 268,270,272 is electricallyconnected between the corresponding one of the three outputs 254,256,258of the WYE connected current transformer 252 and a corresponding one ofthe three discrete inputs 262,264,266 of the full-wave bridge 260. Thethree relay contacts 268,270,272 are normally closed when the switchgeardoor is open. Otherwise, the capacitor 238′ and the electronic circuit240′ can be similar to the respective capacitor 238 and electroniccircuit 240 of FIG. 14.

FIG. 16 shows the trigger circuit 250, which is a three-phase full-timeprotection trigger circuit for the electromagnetic triggers 164,166 ofFIG. 12A. Here, the WYE connected current transformer 252 saturatesabove 10 kA and the capacitor charge rate is about 2 kV/ms. This triggercircuit 250 does employ arc flash light. The WYE connected currenttransformer 252 needs to saturate, because if there is no arc flash, butthere is a fault current, then further charging of the capacitor 278with every half-cycle is not desired. As such, current transformersaturation limits the charging voltage. Here, the sensors 144 of FIG. 10include a light sensor 280. The electronic circuit 282 is structured torespond to a predetermined voltage (e.g., without limitation, about 2kV) across the capacitor 278 and output the current pulse through atriggering mechanism, such as the example expandable electromagnetictriggers 164,166, responsive to arc flash light sensed by the lightsensor 280 when there is also the predetermined voltage across thecapacitor 278.

While specific embodiments of the disclosed concept have been describedin detail, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to those details could bedeveloped in light of the overall teachings of the disclosure.Accordingly, the particular arrangements disclosed are meant to beillustrative only and not limiting as to the scope of the disclosedconcept which is to be given the full breadth of the claims appended andany and all equivalents thereof.

What is claimed is:
 1. An electronic circuit comprising: a number of sensors structured to detect an arc flash from an uncontrolled arcing fault; and a trigger circuit, responsive to the detected arc flash, structured to trigger a triggering mechanism and cause a breakdown of a number of gaps within a low voltage arc flash switch.
 2. The electronic circuit of claim 1 wherein said number of sensors are a plurality of sensors including a number of optical sensors and a number of current sensors.
 3. The electronic circuit of claim 1 wherein said triggering mechanism is an expandable electromagnetic trigger.
 4. The electronic circuit of claim 3 wherein said number of gaps are a plurality of gaps including a first gap and a second gap; wherein said expandable electromagnetic trigger is a conductive ribbon or foil including a first plurality of folds disposable within said first gap and a second plurality of folds disposable within said second gap; wherein each of said first plurality of folds and said second plurality of folds has a compressed position before said conductive ribbon or foil is triggered by said trigger circuit; and wherein each of said first plurality of folds and said second plurality of folds has a triggered position after said conductive ribbon or foil is triggered by said trigger circuit, said triggered position causing the first plurality of folds to expand and breakdown said first gap and the second plurality of folds to expand and breakdown said second gap.
 5. The electronic circuit of claim 4 wherein said breakdown said first gap and said breakdown said second gap occur in about 800 microseconds.
 6. The electronic circuit of claim 4 wherein said conductive ribbon is made of copper and has a width of about 0.1 inch, a thickness of about 0.003 inch and a height of about 0.325 inch.
 7. The electronic circuit of claim 4 wherein each one of both of: said first plurality of folds and said second plurality of folds comprises twelve folds.
 8. The electronic circuit of claim 4 wherein each one of both of said first plurality of folds and said second plurality of folds forms an accordion shape.
 9. The electronic circuit of claim 4 wherein said trigger circuit outputs a current pulse to said conductive ribbon or foil; wherein current flowing through each of said first plurality of folds and said second plurality of folds causes the first plurality of folds to electromagnetically repel each other and causes the second plurality of folds to electromagnetically repel each other, thereby causing said conductive ribbon or foil to move from the compressed position to the triggered position.
 10. The electronic circuit of claim 4 wherein each of said number of gaps is formed by a first electrode separated from a second electrode; wherein said triggering mechanism comprises for each of said number of gaps a U-shaped foil or ribbon conductor including a first end, a first elongated portion, a U-bend, a second elongated portion, an arcuate bend and a second end; wherein said second end is electrically connected to the first electrode; wherein said first elongated portion is parallel to said second elongated portion and separated therefrom by a first insulator; wherein said second elongated portion is parallel to said first electrode and separated therefrom by a second insulator; wherein said triggering mechanism has a compressed position before said triggering mechanism is triggered by said trigger circuit; wherein said triggering mechanism has a triggered position after said triggering mechanism is triggered by said trigger circuit; wherein said first end and said first elongated portion are distal from the second electrode in the compressed position; and wherein said first elongated portion electrically engages the second electrode in the triggered position.
 11. The electronic circuit of claim 10 wherein said U-shaped foil or ribbon conductor is made of copper having a thickness of about 0.003 inch.
 12. The electronic circuit of claim 10 wherein said trigger circuit outputs a current pulse to said U-shaped foil or ribbon conductor; wherein current flowing in opposite directions through the first electrode and the first elongated portion and through the first elongated portion and the second elongated portion causes the first electrode to electromagnetically repel the first elongated portion and causes the first elongated portion to electromagnetically repel the second elongated portion.
 13. The electronic circuit of claim 1 wherein each of said number of gaps is formed by a first electrode separated from a second electrode; wherein said triggering mechanism comprises for each of said number of gaps a foil or ribbon conductor including a first end electrically connected to the first electrode, an elongated portion and a free second end, with a notch formed in the elongated portion proximate the free second end; wherein said elongated portion is parallel to said first electrode and separated therefrom by an insulator in a non-triggered position; wherein said triggering mechanism has a first position parallel to the first electrode before said triggering mechanism is triggered by said trigger circuit; wherein said triggering mechanism has a triggered position after said triggering mechanism is triggered by said trigger circuit; wherein said foil or ribbon conductor is distal from the second electrode in the non-triggered position; and wherein said elongated portion electrically engages the second electrode in the triggered position.
 14. The electronic circuit of claim 13 wherein said trigger circuit outputs a current pulse to or from the free second end and from or to, respectively, the first electrode; wherein current flowing in opposite directions through the elongated portion and the first electrode causes the first electrode to electromagnetically repel the elongated portion, break the elongated portion at the notch and cause the elongated portion to electrically engage the second electrode in the triggered position.
 15. The electronic circuit of claim 13 wherein said number of gaps is two gaps; wherein said triggering mechanism comprises for each of said two gaps a triggering member; and wherein said trigger circuit outputs a current pulse in parallel to the trigger member for each of said two gaps.
 16. The electronic circuit of claim 1 wherein said number of sensors comprises a current sensor; and wherein said trigger circuit comprises: a full-wave bridge including an output and an input electrically connected to the current sensor, a capacitor electrically connected to the output of the current sensor, and an electronic circuit structured to respond to a predetermined voltage across said capacitor and output a current pulse through said triggering mechanism.
 17. The electronic circuit of claim 16 wherein said current sensor is structured to charge said capacitor at a charge rate of about 2 kV/ms; wherein said predetermined voltage is about 2 kV; and wherein said capacitor is charged to the predetermined voltage in about 1 ms.
 18. The electronic circuit of claim 17 wherein said triggering mechanism is structured to breakdown said number of gaps in about 0.4 ms responsive to the current pulse through said triggering mechanism.
 19. The electronic circuit of claim 16 wherein a relay contact is electrically connected between the current sensor and the input of said full-wave bridge; and wherein said relay contact is normally closed when a switchgear door is open.
 20. The electronic circuit of claim 16 wherein said current sensor is a single current transformer structured to sense current flowing in a single phase of switchgear.
 21. The electronic circuit of claim 16 wherein said current sensor is a WYE connected current transformer including three outputs and being structured to sense currents flowing in three phases of switchgear; and wherein the input of the full-wave bridge is three discrete inputs each electrically connected to a corresponding one of the three outputs of said WYE connected current transformer.
 22. The electronic circuit of claim 21 wherein each of three relay contacts is electrically connected between the corresponding one of the three outputs of the WYE connected current transformer and a corresponding one of the three discrete inputs of the full-wave bridge; and wherein said three relay contacts are normally closed when a door of said switchgear is open.
 23. The electronic circuit of claim 21 wherein said number of sensors further comprises a light sensor; and wherein said electronic circuit is further structured to respond to the predetermined voltage across said capacitor and output the current pulse through said triggering mechanism responsive to arc flash light sensed by said light sensor.
 24. A low voltage arc flash system comprising: a low voltage arc flash switch including a number of gaps within said low voltage arc flash switch; a number of triggering mechanisms, one for each of said number of gaps; and an electronic circuit comprising: a number of sensors structured to detect an arc flash from an uncontrolled arcing fault, and a trigger circuit, responsive to the detected arc flash, structured to trigger said number of triggering mechanisms and cause a breakdown of the number of gaps within said low voltage arc flash switch.
 25. The low voltage arc flash system of claim 24 wherein said electronic circuit is disposed on, at or near said low voltage arc flash switch. 